This invention relates to a process for improving the solution viscosity and physical property stability upon aging of silicone pressure-sensitive adhesives containing residual silanol groups by endblocking such silanol groups with triorganosilyl endblocking units. This invention also relates to the silicone pressure-sensitive adhesive compositions obtained by such a method.
Silicone pressure-sensitive adhesives are commonly prepared by blending copolymeric resins composed of triorganosiloxy units and SiO.sub.4/2 units with silanol-endblocked, polydiorganosiloxanes as shown in U.S. Pat. Nos. 2,736,721 (Dexter, issued 2/28/56) and 2,814,601 (Currie, et al, issued 11/26/84). Such blends, which are usually a solvent solution of the resin and polydiorganosiloxane, tend to increase in viscosity upon aging and the tack and adhesion values of films made from the pressure sensitive adhesives can also change with time. Such changes are not desirable.
Catalysts are often added to improve the physical properties of the silicone pressure-sensitive adhesive film. In addition to free radical polymerization catalysts such as peroxides, silanol condensation catalysts such as organometallic compounds and metal salts of carboxylic acids (see the Dexter Patent, above) and amines (see Canadian Pat. No. 575,664, Bartell, issued 5/12/59) have been suggested as curing catalysts for the heat cure of such films. Condensation cure is effected through the residual silanol radicals present in the resin copolymer which can contain up to about 3-4 weight percent of hydroxyl radicals bonded to silicon atoms based on the total weight of the copolymer resin ("silanol content"). The silanol content of the silanol-endblocked polydiorganosiloxanes will vary with the molecular weight of the polydiorganosiloxane present in the silicone pressure-sensitive adhesive blend. The presence of silanol condensation catalyst that was deliberately added or else is present in the resin copolymer or polydiorganosiloxane, e.g., an acid, because such catalysts were used in their manufacture can cause undesirable slow silanol condensation at room temperature with a resulting change in solution viscosity upon aging and can also result in changes in the pressure-sensitive adhesive film properties. Additives such as isopropanol have been used to improve the viscosity stability of blends of silicone copolymer resins and polydiorganosiloxanes as noted in U.S. Pat. No. 4,309,520 (Blizzard, issued 1/5/82), but the viscosity still changes upon aging.
Other silicone pressure-sensitive adhesive systems utilizing a cure system involving the platinum-catalyzed addition of silicon-bonded hydrogen atoms to silicon-bonded vinyl radicals have been taught such as in U.S. Pat. No. 3,983,298 (Hahn, et. al., issued 9/28/76). While Hahn uses a vinyl-endblocked polydiorganosiloxane as one component of his pressure sensitive adhesive compositions, the silicone resin copolymer employed therein can have a silanol content of as much as 3 to 4 percent by weight which, upon condensation, can result in a change in the stability of the viscosity and physical properties of the pressure-sensitive adhesive film upon aging.
Preparation of silicone pressure sensitive adhesives by the intercondensation of silanol-containing silicone copolymer resins with silanol-endblocked polydiorganosiloxanes by heating the mixture in the presence of a silanol condensation catalyst until the desired physical property characteristics are obtained is taught in U.S. Pat. Nos. 2,857,356 (Goodwin, issued 10/21/58); 3,528,940 (Modic, issued 9/15/70) and 3,929,704 (Horning, 12/30/75) and Canadian Pat. No. 711,756 (Pail, issued 6/15/65). This process still results in a product which contains residual silanol radicals which can condense and thereby affect the stability of the pressure-sensitive adhesive. The Pail Patent reacts silanol-functional silicone copolymer resins with low molecular weight silanol-endblocked polydiorganosiloxanes in the presence of an amine catalyst until the desired degree of adhesiveness is obtained.
Silicone pressure-sensitive adhesives have been made wherein the silicone copolymer resin has been further treated with a endblocking agent such as hexamethyldisilazane to reduce the silanol content of the copolymer to less than 1 percent by weight in U.S. Pat. No. 4,255,316 (Blizzard, issued 3/10/81). The '316 patent mentions that such reduced silanol content resin copolymers can be made with polydiorganosiloxanes having oxygen bonded terminating radicals such as H--, A'--, or A'.sub.3 Si-- where A' is a monovalent hydrocarbon radical having from 1 to 6 inclusive carbon atoms; the oxygen-bonded terminating radicals are preferably H-- or A'-- to provide a reaction site for curing the fluid. The Blizzard '520 patent mentions the use of a pressure-sensitive adhesive formulation as a control material which was a cold mix of a silanol-endblocked polydiorganosiloxane gum with a blend of two silicone copolymer resins: a major portion of which had about 2 percent silanol content and a minor portion of a resin copolymer which had been treated with hexamethyldisilazane to provide a maximum of about 0.4 percent silanol content. The aforementioned '316 and '520 patents could still conceivably contain residual silanol groups which could affect stability upon aging.